1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to disk drives that vary the timing of the servo detect window according to measured variations in the angular velocity of the drive's spindle motor.
2. Description of the Prior Art
In contemporary magnetic hard disk drives, servo sectors of servo information are interspersed with data sectors circumferentially in concentric tracks around the surfaces of the disk or disks. As shown in FIG. 1, the format of a track of a rigid magnetic disk drive calls for regularly spaced embedded servo sectors (also called servo wedges) (of which representative servo sectors 121, 122, 123 are shown) containing servo information therein. Between the embedded servo sectors are a number of data sectors. There may be an integer number of data sectors between any two servo sectors. However, a track may also include a non-integer number of data sectors between adjacent servo sectors, as some of the data sectors may be split across servo sectors.
As shown in FIG. 2, a servo sector (exemplary servo sector S2 122 being shown for illustrative purposes only, it being understood that all of the servo sectors shown in FIG. 1 have the same format) may include a preamble 201, 202, a servo sync word 203, a track identifier 204 and servo burst fields A, B, C and D, referenced by numerals 205, 206, 207 and 208, respectively. The preamble 201, 202 may, for example, include a first portion 201 that includes a fill code to enable the read channel to adjust is gain and allows a phase locked loop (PLL) to achieve bit synchronization with the incoming servo information. The preamble may also include a second portion 202 that includes a DC erase portion in which there are no logical transitions (such as an uninterrupted string of zeros, for example) for a specified length. As areas containing no transitions are illegal everywhere else on the disk, the DC erase field uniquely identifies this portion as being part of a servo sector. Following the preamble 201, 202 is a servo sync word 203 that identifies the sector as being a servo wedge and that establishes byte synchronization. After the servo sync word 203 is a track identifier (ID), which uniquely identifies the number of the track being read. The servo burst fields 205, 206, 207 and 208 are used to determine the head's current location on the track. The detection of the servo sync word establishes a positive timing reference and allows a disk controller integrated circuit to forecast the timing of the next servo sync word, since they are equally spaced along the tracks recorded on the recording surface of the disk or disks and since the nominal angular velocity of the disk is known.
A spindle motor drives the disk or disks in rotation. However, the spindle motor does not, in practice, drive the disks at a perfectly constant angular velocity. Indeed, there are some variations in the angular velocity at which the spindle motor drives the disk in rotation. Such variations may cause the detection of the servo sync words to be read later (in the case wherein the spindle motor has slowed down relative to its nominal speed) or earlier (in the case wherein the spindle motor is driving the disk faster than its nominal angular velocity) than forecast.
The disk controller, to detect the servo sync word, opens up a timing window during a servo gate signal (not shown), which enables the read channel to begin the detection of the preamble of the servo sector, in order to establish gain, achieve bit synchronization and the like. The controller also opens another timing window for the detection of the servo sync word. This timing window is made as narrow as possible within the constraints of the variations in the angular velocity of the spindle motor, to reduce the possibility of a false detection of the servo sync word. If the servo sync word is made unique and the timing window for its detection is made as narrow (is opened only for a short duration), the probability of a false detection is relatively low.
During most of the operation of the drive, the controller operates in hard sector mode. In hard sector mode, upon detection of a servo sync word, a fixed interval is established for the detection of the next servo sync word in the next servo sector. After the fixed interval has elapsed, the controller opens up the window to detect the next servo sync word. The fixed interval may be established by a counter that is decremented until it reaches zero, for example. If the drive has, for any reason, lost synchronization or is just starting up and has not yet achieved synchronization, the drive may operate in a soft sector mode. In soft sector mode, the controller may not know the location of the read/write heads over the disk. In soft sector mode, the servo gate signal is opened up and the drive looks for the next preamble, track ID and servo sync word. Once several servo sync words have been detected and synchronization is achieved, the drive may switch to hard sector mode.
The detection of the servo sector and of the servo sync word within the servo sector, therefore, is essential as it allows the controller to predictably determine the start of the data sectors on the track.
FIG. 3 is a diagram of an exemplary structure of a data sector recorded on the recording surface of a disk of a magnetic hard disk drive. Exemplary data sector 104 (exemplary data sector D4 104 being shown for illustrative purposes only, it being understood that all of the data sectors shown in FIG. 1 have the same format) includes a preamble 301, 302, followed by data sync word 303 and the data 304. The read channel decodes each data sector and the data sync word establishes when the data 304 may be clocked in (writing) or out (reading). The aforementioned speed variations in the angular velocity of the spindle motor require that guard bands 130 (FIG. 1) be included before each data sector. The width of the guard bands is a function of the variation in spindle motor speed between writing data to that sector and subsequently reading the data back. Indeed, the width of the guard band is related to the minimum to maximum spindle motor speed variation, as well as the time interval between re-synchronizing the sector timing logic upon detection of the servo sync word. As the read/write head moves away from the detected servo sync word, the uncertainty (it is a sampled system) of the location of a sector is the difference between the actual angular velocity of the spindle motor and the nominal speed thereof, and increases in linear fashion. This results in maximum uncertainty just before the detection of the next servo sync word. The wider the guard bands are, the more disk real estate is made unavailable for storing data. Therefore, if the width of the guard bands could be reduced, comparatively more data could be stored on the disks.
What is desired, therefore, are disk drives that enable the width of the guard bands to be decreased and the capacity of the disk media to be increased, given the constraints imposed by the aforementioned variations in the angular velocity of the spindle motor.